MEMS microphones generally have a capacitor comprising at least one stationary electrode embodied in a planar fashion and having relatively high stiffness, the so-called backplate, and a further electrode embodied in a planar fashion and arranged parallel, the elastic and able to oscillate (oscillatory) membrane (also called diaphragm). In addition, the capacitor can have even further stationary or oscillatory electrodes.
If sound impinges on the capacitor, the one or a plurality of membranes are excited to effect oscillations corresponding to the sound relative to the one or the plurality of backplates. On account of the temporally varying spacings of these capacitor electrodes, the capacitance of the capacitor varies. Evaluation electronics convert the sound-induced capacitance changes into an electrical signal which can be processed further by a circuit environment.
On the one hand, there are MEMS microphones having at least one freely movable membrane. Said membrane is surrounded by mounts such that it can oscillate freely. However, a change in position is possible only to a limited extent. Figure ii shows a schematic illustration of such a MEMS microphone.
On the other hand, there are MEMS microphones having at least one membrane which is anchored such that it can oscillate but substantially cannot alter its position. FIG. 1 shows a schematic illustration of such a MEMS microphone.
It holds true for both types of microphones that their sensitivity and their signal quality can be improved by an acoustically active region of the capacitor having the largest possible area, relative to the acoustically inactive region. This is because the acoustically inactive region of the capacitor contributes to a parasitic capacitance which impairs the signal quality.
In this regard, the document WO2013/071951, for example, discloses MEMS microphones having a reduced parasitic capacitance of the capacitor formed by a membrane and a backplate.
A back volume that is as large as possible is likewise advantageous. In this case, the back volume is the volume which is separated from the surroundings and which is situated behind the capacitor in the sound direction.
However, it is necessary to construct MEMS microphones such that they are as small as possible, which is therefore detrimental to the signal quality.
The LLF (lower limiting frequency) is that frequency which is defined by the 3 dB limit at the lower end of the acoustic frequency band in which a microphone operates. It therefore determines the lower limit of the operating range and is approximately 20 Hz in the case of MEMS microphones having a flat frequency response in the audio range. An LLF that is as small as possible is therefore desirable. A reduced LLF can be achieved by enlarging the back volume, which is difficult to implement on account of the endeavors to achieve ever more extensive miniaturization. A reduced LLF can also be achieved by lengthening the ventilation path which enables a slow pressure equalization between the back volume and the surroundings, which is prevented on account of constraints in production steps.